Parametric nuclear quadrupole resonance spectroscopy system and method

ABSTRACT

A system and method for probing a specimen to determine one or more components by utilizing a first signal to excite the specimen at a nuclear quadrupole resonant frequency and observing changes in a specimen property. One exemplary property may be dielectric constant. Another exemplary property may be magnetic permeability. In one embodiment, the first signal is unmodulated and a second signal is observed for the presence of modulation at the frequency of the first signal. Alternatively, the first signal may be modulated and the second signal may be observed for the presence of the modulation. A system is disclosed wherein the specimen is excited using the first frequency and a radar at the second frequency is used to observe changes in radar reflectivity of the specimen due to the excitation.

RELATED APPLICATIONS

This application claims the benefit under 35 USC 119(a) of U.S.Provisional application 60/638,858 filed Dec. 22, 2004 by Fullerton,which is hereby incorporated herein by reference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention pertains to the field of materials detectionand/or identification, more particularly to the field of detection ofmaterials using RF probing energy.

2. Background of the Invention

Nuclear Quadrupole Resonance (NQR) is a well-known spectrographictechnique that is used to detect and identify molecular structures bythe characteristic NQR of atomic species contained within. Certainatoms' nuclei have the characteristic of absorbing RF energy whenexposed to a frequency that causes its nucleus' spin axis to hop betweenseveral stable orientations. This is possible only if the particularnucleus has a non-symmetrical charge distribution that permitsinteraction with the atom's electron cloud non-symmetries. Thecomplexity of these quasi-stable orientations typically leads to aseries of closely spaced, narrow line width absorption lines. An exampleof such a nucleus is the common isotope Nitrogen-14.

This characteristic resonance has been used commercially to positivelydetect substances such as explosives contained within shippingcontainers. The method used is to sweep a local RF field through thefrequencies of interest and, by using a bridge structure, measure theloading on the RF source as it passes through the resonances and usethis information to identify the material under observation.

There are several technical difficulties with this method that limitsits usefulness to very short range applications on the order of onemeter, whereas it would be very useful to provide much greater rangewhen dealing with materials such as explosives, i.e. tens of meters orfurther of standoff would be desirable.

Issues with the conventional NQR spectrographic techniques concern theability to measure very small signals buried within very large ones, andthe directionality of the frequencies used to stimulate the specimenunder study.

There are two principal conventional NQR techniques. The first onerelies on detecting a target's absorption of resonant radio frequencyenergy from an interrogator tuned to the specimen's resonant frequency.This is usually accomplished with a bridge configuration in which thevoltage across a coil of wire acts as an exciter at the interrogatingfrequency and that voltage is nulled in a bridge circuit at a frequencythat is near an expected resonance but not at it. When the exciter'sfrequency is tuned past a specimen's resonance, a signal appears in onearm of the bridge whose amplitude indicates the degree of absorption bythe specimen. A problem with this approach is that the circuit impedancevaries with frequency even if there is no resonance due to the reactiveelements used in the exciter, and the RF impedance of the specimen mayalso vary as the frequency is swept during analysis. These effectsconspire to limit the sensitivity of a spectrometer using this approach,a limitation that is typical of any spectrometric technique that usesthe same frequency to interrogate as that to which the detector issensitive. Examples of spectrometers for which this is not so are Ramanand florescence spectroscopy, in which the illumination frequency isvery different than the response of the specimen, so the illuminator caneasily be filtered out and the detector can therefore be made verysensitive to the alternate frequency since it is not blinded by itsinterrogator.

The second major type of conventional NQR spectrometer is based onproducing an echo from the sample. In this method the interrogator emitsa pulse of resonant frequency energy toward a specimen under study, andthen its receiver listens for immediate re-emission of the samefrequency energy when the interrogator is turned off. This mode is lessprone to blinding by the interrogator but is less sensitive over allbecause of three effects. First, the average energy available to excitethe specimen is lowered by the duty cycle of the system. Second, itsnoise figure is worsened by the necessity to pass wide band RF pulsesthereby increasing the level of thermal noise in the receiver. Thelatter effect is particularly bad since the resonances can havebandwidths of single Hertz while the millisecond time delay of the echomandates a minimum bandwidth of several kilohertz, increasing the noisefigure on the order of 30 dB. The third problem with the echo approachis the very high transmit/receive signal loss. The amount of RF energystored up to be reemitted is quite small, so it is a weak signal tostart with.

All of the resonances of nuclei that are of interest occur below about1GHz. Nuclei that resonate toward the high end of this range may benefitfrom directional interrogators but many important species occur in thehundreds of KHz to a few 10's of MHz. Nitrogen 14 is particularlyimportant to the detection and identification of hidden explosives andthese are in the few MHz range and below. Antennas that operate in thisrange are either very large or will experience very poor directionality.Thus, even if a stand-off system is constructed that overcomes thesensitivity limitations previously described it will typically not beable to resolve its location in the field with sufficient accuracy tolocate it.

Atmospheric and man-made noise in this frequency range is far abovethermal limits so even if the system is ideally constructed itssensitivity may be further limited by ambient rather than system noise.

These factors combined yield a very short range of operation, andcommercial systems therefore operate in near proximity to the target, onthe order of 1 meter or less range. This range is entirelyunsatisfactory for many applications such as locating hidden explosivesthat are connected to trip wires.

Thus, there is a need for a non-destructive system for detecting andidentifying materials, where the system may be used at a distance fromthe material and may incorporate sufficient directionality to locate thematerial within the range of the system.

BRIEF SUMMARY OF THE INVENTION

Briefly, the present invention is a system and method for probing aspecimen to determine one or more components by utilizing a first signalto excite the specimen at a nuclear quadrupole resonant frequency andobserving changes in a specimen property. One exemplary property may bedielectric constant. Another exemplary property may be magneticpermeability. In one embodiment, a second signal is used to probe thematerial to observe changes in the property.

In one embodiment, multiple frequencies may be excited separately orotherwise to further determine one or more components of the specimen.

In one embodiment, the first signal is unmodulated and the second signalis observed for the presence of modulation at the frequency of the firstsignal.

Alternatively, the first signal may be modulated and the second signalmay be observed for the presence of the modulation.

A system is disclosed wherein the specimen is excited using the firstfrequency and a radar at the second frequency is used to observe changesin radar reflectivity of the specimen due to the excitation. In oneembodiment the first frequency is modulated and the radar return signalis processed using synchronous processing to detect the modulation onthe radar return signal.

The system may be used to detect materials by utilizing probe signalenergy transmitted through the material, scattered by the material orreflected from the material.

BRIEF DESCRIPTION OF THE FIGURES

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements. Additionally, the left most digit(s) of areference number identifies the drawing in which the reference numberfirst appears.

FIG. 1 depicts an exemplary calculation of the angle modulation of aprobe signal when the dielectric constant of the material is modulated1.0% by an excitation field in accordance with the present invention.

FIG. 2 illustrates an exemplary system for exciting the material anddetecting the modulated reflection signal in accordance with the presentinvention.

FIG. 3 illustrates the system of FIG. 2 further including the use of themodulation signal information to process the received probe signal.

FIG. 4 illustrates a system that utilizes an unmodulated excitationsignal.

FIG. 5 illustrates further details in one embodiment of the receiverdemodulator.

FIG. 6 illustrates a PNQR system based on an Ultra Wideband Radar.

FIG. 7 shows an alternative embodiment wherein the probe signal passesthrough the material in one direction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention discloses a method that overcomes many of theshortcomings of the conventional methods of Nuclear Quadrupole Resonance(NQR) spectroscopy. The invention uses two separate signals, the firstsignal at one of the resonant frequencies of the specimen and the secondsignal at a frequency that responds to changes in either the specimen'sdielectric constant, magnetic permeability or other related property,which are induced by the first signal. Since the principle of operationis based on a parametric modulation or variation, the process of thepresent invention may be termed Parametric Nuclear Quadrupole Resonance(PNQR) spectroscopy.

The basic principle of PNQR spectroscopy is the detection andmeasurement of the interaction between the quadrupole electric field ofan atom's nucleus and its own valence electrons. This interaction ismaximized when an exciting radio frequency field's frequency is equal toa resonance of this interaction in a given atom. The charge distributionand electric polarization of the valence electrons in a molecule are thesource of the dielectric constant of the material, which relates to theenergy that is stored in a volume of material when it is exposed to agiven electric dipole field. The dielectric constant is also responsiblefor determining the speed that light and radio waves propagate throughthe material. When a material is excited at its resonant frequency by anexternal radio frequency (RF) field, the resonance response will alsohave the effect of modulating the macro dielectric constant of thematerial. The change in dielectric constant may be observed, however, atother frequencies. In particular, the speed of propagation of otherfrequencies may be modified. Therefore when the material is interrogatedwith a signal at a different and non-resonant radio frequency by passingthe signal through the material, at the same time that it is beingexcited with a signal at the resonant frequency, then the non-resonantsignal will be phase modulated (as a result of the varying delay) as itpasses through the material. Note that the material may be interrogatedby the non-resonant frequency signal by receiving any portion of theprobe signal that is influenced by the bulk of the material. Thus thesignal may pass through the material in one direction, or be reflectedor partially reflected, or be scattered by the material. In all thesecases, the signal propagation is influenced by the material inaccordance with the material dielectric constant and the signalpropagation will be influenced by changes in the dielectric constantinduced by the excitation field.

To summarize, a material to be analyzed is excited with a first RF fieldthat has frequency components at the NQR frequency of the material. Thesame material is also illuminated by second RF signal at a differentfrequency, and the phase modulation of the latter signal, after passingthrough the sample, is analyzed for modulation by the first RF field.

In a further aspect of the invention, the sensitivity may be furtherimproved by modulating the exciting signal and detecting the modulationon the second signal. This aspect may be better understood withreference to exemplary embodiments. In a first exemplary embodiment, thefirst RF signal may be a 3 MHz signal broadcast by a vertical dipoleantenna, and 100% amplitude modulated by a 1 KHz sine wave. The secondsignal may be a 10 GHz signal that is scattered off of the material, ina manner similar to a radar. The second signal may be transmitted via adirectional antenna such as a parabolic dish or a helical antenna. Whenthe 10 GHz signal is directed at the material some of the energy maypenetrate the material, traveling through the material, reflecting fromone or more boundaries, and traveling back through the material, andpropagated back to the radar receiver. The reflected 10 GHz signal isthen analyzed for evidence of the 1 KHz modulation. In contrast to theconventional prior art the exemplary system presents a number ofbenefits:

-   -   1. Both man-made and atmospheric noise is negligible at 10 GHz,        so operation is only limited by good receiver design and the        thermal noise of the receiver.    -   2. Since both the first signal and second signal may use CW        carriers (100% duty cycle) the power on target is maximized with        resulting sensitivity improvement.    -   3. The target's low frequency resonance (e.g. 3.41 MHz) is        parametrically converted to the microwave regime (e.g. 10 GHz)        in this example so transmit/receiver isolation is very good and        again system sensitivity is maximized.    -   4. The microwave frequency of the second signal (e.g. 10 GHz)        permits high directivity in a portable package so that materials        such as hidden explosives can be readily located.    -   5. The modulation frequency is offset from the microwave carrier        frequency of the second signal, which further improves the        sensitivity and dynamic range of the receiver.

In an alternative embodiment, the first signal is unmodulated and thereflected second signal is analyzed only for modulation effects at theactual RF frequency of the first signal.

The present invention may be better understood with reference to theFigures. FIG. 1 depicts an exemplary calculation of the angle modulationof a probe signal when the dielectric constant of the material ismodulated 1.0% by an excitation field in accordance with the presentinvention. FIG. 1 is based on a 1.5 cm thick slab of material whosedielectric constant of 3.5 is modulated 1% by a PNQR bias signal at 3.41MHz. FIG. 1 shows the relative phase of a 10 GHz probe signal reflectedfrom the material. Referring to FIG. 1, vector 102 represents areference reflection from the front surface of the material. Vector 104represents the relative angle of a reflection from the back surface ofthe material with no 3.41 MHz excitation present. Vector 106 representsthe relative angle of the reflection from the back surface of thematerial with 3.41 MHz excitation present. The difference between vector104 and vector 105 is shown as vector 108, which is the modulation dueto the 3.41 MHz excitation. The phase shift results from the propagationdelay through the material, which in turn is a function of thedielectric constant. As the dielectric constant is increased, the delayis increased resulting in a phase shift of the RF signal propagatingthrough the material. Thus, the phase shift is increased by increasingthe thickness of the material, or increasing the RF probe frequency, orincreasing the amplitude of the excitation frequency.

In the example shown in FIG. 1, the difference vector 108 is 24 dB belowthe reflection signal, vector 104. With a probe signal of 30 GHz, thesame signal delay would produce a larger phase delay, resulting in adifference vector of 15 dB below the reflection, improving detection by9 dB. Likewise, a probe signal of 60 GHz would result in a differencevector 9 dB below the reflection. In a similar manner, a thickermaterial cross section would also increase the modulation. For example amaterial cross section of 3 cm would result in a difference vector 18 dBbelow the reflection at 10 GHz, a six dB increase from the 1.5 cmthickness.

FIG. 2 illustrates an exemplary system for exciting the material anddetecting the modulated reflection signal in accordance with the presentinvention. Referring to FIG. 2, a first RF frequency source 202 is tunedto a resonant frequency of interest for a material of interest. Forexample the-frequency may be 3.410 MHz, which is a frequency fornitrogen, which is abundant in typical explosives. The RF signal fromthe first RF frequency source 202 is modulated 206 by a modulationsignal 205 from a modulation frequency source 204. For example themodulation may be a 1 kHz sine wave producing 100% AM modulation.Alternatively, the modulation may be a square wave, producing on-offcarrier modulation or another waveform, as desired. The modulated firstRF signal is amplified 208 and transmitted via an antenna 210. The firstRF signal (also called excitation signal or bias signal) may betransmitted omni directionally, in part because short range directionaltransmission of frequencies on the order of a few megahertz is typicallyimpractical, however directionality may be used if desired, and may bemore practical at the higher excitation frequencies. An omni directionalantenna 210 is depicted in FIG. 2. The excitation signal impinges on thespecimen 212 and modulates the dielectric constant as described.

A second RF frequency source 214 and amplifier 216 generate a second RFsignal (also called a probe signal,) typically at a microwave frequency.The probe signal is directed to the specimen 212 using a directionalantenna 220. A reflected signal from the specimen is then received bythe directional antenna 220, coupled through a duplexer or TR Switch218, and processed by a receiver 222 to detect variations in thereflected signal due to the modulation of the dielectric constant of thespecimen 212. The detected modulation level may then be presented as aspectrometer output 226. The probe transmitter and receiver are similarin functionality to a radar. A number of techniques may be usedincluding systems similar to CW Doppler radar, pulsed radar, and UWBradar. A particular adaptation of the radar is to process the receivedsignal to detect slight phase variations that vary in accordance withthe modulation. In one embodiment, a filter tuned to the modulationfrequency is used. In another embodiment, the modulation signal itselfis used to coherently, or synchronously process the received signal todetect the induced variations. In another embodiment, a UWB radar isscanned over a range of distances in discrete steps. At each step, thereturn signal is processed to detect the induced modulation.

FIG. 3 illustrates the system of FIG. 2 further including the use of themodulation signal information to process the received probe signal.Referring to FIG. 3, the modulation signal 205 is coupled to thereceiver demodulator 222 to synchronously process the received signal219 to detect variations that are synchronous with the modulation 205.In an alternate embodiment, the receiver demodulator 222 may provide themodulation signal 205 to the modulator 206, or the receiver demodulator222 and modulator 206 may be fed from a timing generator (not shown). Inessence, the modulator 206 and receiver demodulator 222 operate inidentical phase and frequency to achieve the synchronous (coherent)processing. In one embodiment, the received signal 219 may be processedby a multiplier circuit and the modulation signal 205 may be a sinewaveform fed to the modulator 206 and to the receiver 222. In anotherembodiment, the receiver circuit 222 may operate on modulation polarityinformation only, i.e. a square wave of +1 and −1 value may be sent tothe receiver 222 to reverses the polarity of the received signal 219 inphase with the modulation 205 to detect variations that are synchronouswith the modulation 205. The synchronous processing may be followed by alow pass filter with a bandwidth of only a few Hertz to maximize signalto noise and improve the range of the system.

FIG. 4 illustrates a system that utilizes an unmodulated excitationsignal 402. Referring to FIG. 4, a continuous unmodulated excitationsignal 402 is transmitted by the excitation transmitter 208. The proberadar generates a probe signal and receives a reflection. The receiverprocessor 222 synchronously detects induced variations of the receivedsignal 219 at the excitation frequency 202.

FIG. 5 illustrates further details in one embodiment of the receiverdemodulator. Referring to FIG. 5, the receiver demodulator 222 firstcoherently detects the received signal 219 at detectors 504A and 504B byusing the transmitted signal frequency information 215 to producereceived signals 506A and 506B at a first baseband. The received signalsmay then be filtered if desired (not shown). The received signals arethen further coherently processed at multipliers 508A and 508B using themodulation frequency information 205. Note also that the probetransmitter signal 215 is divided into an in-phase (I) and quadraturephase (Q) version to produce an in-phase 506B and quadrature 506Abaseband to insure that the signal 219 will always be detected in onechannel 504A or the other 504B, irrespective of the actual incoming RFphase of the signal 219.

The modulation signal 205 is used to multiply the received I 506B and Q506A signals to yield detection outputs 226A and 226B. The detectionoutputs are fed to a processor 502 to generate the final detectionsignal 510. The I 226A and Q 226B detection outputs may be filtered witha low pass filter (part of processor 502). The bandwidth of the filtermay be selected to trade the benefits of low signal to noise with thebenefits of fast response. The I 226A and Q 226B outputs may be combinedfor a combined response output 510. The combination function may beideally the square root of the sum of the squares of the two signalvalues, or may be a simpler absolute value summation or a logic ORfunction combining a threshold detection on each signal 226A or 226B.

FIG. 6 illustrates a PNQR system based on an Ultra Wideband Radar.Referring to FIG. 6, a precision timer 602 provides timing signals 604to a transmit pulser 608, which generates and transmits UWB pulses. Theprecision timer 602 also provides timing signals 606 to a receiver delay610 that provides the proper delay for receiving the reflected UWBpulses 219. The receive delay 610 is typically scanned in steps over arange of delays corresponding to a span of distances from the proberadar. At each distance, the received pulses 219 are coherently detected504 to produce a baseband signal 506. The baseband signal 506 is thenfurther processed by coherent processing 508 with respect to themodulation signal 205. In the embodiment shown, the modulation signal205 and the coherent processing signal 205 are provided by the precisiontimer 602. The baseband signal 506 is multiplied by the coherentprocessing signal 205 to yield the spectrometer output 226, which may befurther processed by a processor 506, which may include low passfiltering or thresholding as needed for the application.

Advantages of the UWB system include two dimensional location, i.e.,location can be further defined in range as well as angle. Further,given a known (measured) range and a detected signal magnitude, theamount of material may be estimated.

In another embodiment, other techniques such as chirped, swept, or PNcoded signals may be used to provide range resolution.

FIG. 7 shows an alternative embodiment wherein the probe signal passesthrough the material in one direction. For some applications, such asfor example, but not limited to, inspecting articles on an assemblyline, or inspecting luggage at an airport, it may be desirable toutilize a pass through arrangement wherein the RF probe signal is passedthrough the material rather than reflected from or scattered by thematerial.

Applications

The invention has applications in commercial, industrial, mining,survey, security, military, and other. The system may be used to searchfor minerals, inspect items on a production line, or measure the mix ofingredients in chemical processes, or look for veins of ore, inspectairline baggage or passengers, look for roadside improvised explosivedevices, de-mining farmland and other applications where materialdetection and/or identification is needed.

The system may be made portable or mobile for survey type applications,or may be made fixed for industrial process applications or securitymonitoring.

In operation, the system may typically tune the excitation frequency toa number of resonant frequencies in the desired material and evaluateeach frequency to more positively determine the material or may tune tothe frequencies of different materials to help distinguish betweendifferent materials and reject false alarms.

Variations

The functional elements described herein may be implemented in digitalor analog circuits or a combination. A computer or processor may includeassociated function blocks and may be implemented using a programmablecomputer, digital logic, state machine, signal processor or digitalcontroller technology. Transceiver function blocks shown separate fromthe processor may be implemented within a physical processor in a givenimplementation.

Conclusion

Thus described is a non-destructive system for detecting and identifyingmaterials, where the system may be used at a distance from the materialand may incorporate sufficient directionality to locate the materialwithin the range of the system.

While particular embodiments of the invention have been described, itwill be understood, however, that the invention is not limited thereto,since modifications may be made by those skilled in the art,particularly in light of the foregoing teachings. It is, thereforecontemplated by the appended claims to cover any such modifications thatincorporate those features or those improvements which embody the spiritand scope of the present invention.

1. A method for identifying a material comprising the steps of: excitingthe material with an excitation signal, said excitation signal adjustedto a nuclear quadrupole resonance frequency of said material;determining a change in a property of said material, said changeresulting from the excitation signal; and identifying the material basedon said change in said property wherein said property is a dielectricconstant, or a magnetic permeability.
 2. A method for identifying amaterial comprising the steps of: exciting the material with anexcitation signal, said excitation signal adjusted to a nuclearquadrupole resonance frequency of said material; determining a change ina property of said material, said change resulting from the excitationsignal: and identifying said material based on said change in saidproperty, wherein said property is a propagation delay through saidmaterial.
 3. The method of claim 2, wherein the excitation signal ismodulated and the change in said material property is determined bydetecting a modulation of the propagation delay through the materialresulting from the modulation of the excitation signal.
 4. The method ofclaim 3, wherein the propagation delay is determined by propagating aprobe signal through the material and measuring a delay of said probesignal resulting from said excitation signal.
 5. The method of claim 3,wherein determining the propagation delay includes the steps of:directing a probe signal toward the material; detecting probe signalenergy reflected from said material; measuring a signal delay in saidreflected probe signal energy, said signal delay resulting from saidexcitation signal.
 6. The method of claim 5 wherein the step ofmeasuring a signal delay in said reflected probe signal energy includesthe step of: coherently processing said reflected probe signal energybased on said modulation of said excitation signal.
 7. The method ofclaim 6 wherein the coherent processing of said reflected probe signalenergy is based on the phase of said modulation of said excitationsignal.
 8. A system for identifying a material, said system comprising:a first transmitter, transmitting an excitation signal to said material,said excitation signal being tuned to a nuclear quadrupole resonancefrequency of said material; a second transmitter, transmitting a probesignal to said material; a receiver receiving probe signal energy fromsaid material, said receiver generating a baseband signal; and aprocessor, said processor detecting a change in said baseband signal,said change resulting from said excitation signal.
 9. The system as inclaim 8, wherein said received probe signal energy is scattered from,reflected by, or transmitted through said material.
 10. The system as inclaim 8, wherein said probe signal is continuous wave or is pulsed. 11.The system as in claim 8, further including a modulator, wherein theexcitation signal is modulated by the modulator.
 12. The system as inclaim 11, wherein said processor synchronously detects said receivedsignal with respect to said modulation.
 13. The system as in claim 12,wherein said processor synchronously detects said received signal withrespect to the phase of said modulation.
 14. The system as in claim 12,wherein said processor coherently detects said received signal withrespect to the waveform of said modulation.
 15. The system as in claim8, wherein the probe signal is an Ultra Wideband signal.
 16. The systemof claim 15, further including Ultra Wideband distance determinationcapability, wherein the distance of said material is determined usingthe Ultra Wideband signal.
 17. The system as in claim 8, wherein theexcitation signal is tuned to each of a plurality of nuclear quadrupoleresonant frequencies of said material and said processor detects saidchange in said baseband signal for each of said plurality of resonantfrequencies.
 18. A system for identifying a material comprising: a firsttransmitter for exciting said material at a nuclear quadrupole resonantfrequency of said material; a second transmitter for directing a probesignal toward said material; a receiver for receiving a reflected probesignal from said material; a processor for detecting a change in aproperty of the reflected signal resulting from said excitation of saidmaterial.